Charge/discharge control method and charge/discharge control apparatus for lithium ion battery

ABSTRACT

In a charge/discharge control method of a lithium ion battery having a negative electrode active material and connected to a charge/discharge control device, battery information regarding a charge/discharge state of the lithium ion battery is acquired by the charge/discharge control device, a degradation state of the lithium ion battery is determined on the basis of the battery information, by the charge/discharge control device, and a voltage range for charge/discharge of the lithium ion battery is changed on the basis of a determination result of the degradation state, by the charge/discharge control device.

TECHNICAL FIELD

The present invention relates to a charge/discharge control method and acharge/discharge control device of a lithium ion battery.

BACKGROUND ART

Recently, hybrid vehicles using engines and motors together as powersources and electric vehicles using only the motors as the power sourceswithout having the engines are developed and manufactured, from theviewpoint of environmental protection and energy saving. Secondarybatteries that can charge/discharge electricity repetitively are used aspower supplies (energy sources) of the hybrid vehicles and the electricvehicles and become essential components. Above all, a lithium ionbattery is a secondary battery of a high energy density in which anoperation voltage is high and a high output is easily obtained. For thisreason, recently, the lithium ion battery becomes important increasinglyas the power supply of the hybrid vehicle or the electric vehicle.

Conventionally, technology for using an active material of a largecapacity to realize the high output and the high energy density in thelithium ion battery is known. However, an active material such assilicon and SiO known generally as the active material of the largecapacity is greatly expanded/contracted at the time of charge/discharge.For this reason, if the charge/discharge is repeated in a battery usingthe active material, degradation is likely to occur due to collapse andisolation of the active material and a cycle characteristic is bad.

As technology for resolving the above problem, technology forsuppressing degradation of the active material by using a binder havinghigh coatability for an electrode material and high adhesion isdisclosed in PTL 1.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Publication No. 2013-33692

SUMMARY OF INVENTION Technical Problem

According to PTL 1, the degradation of the active material is suppressedby devising a material of the binder, so that deterioration of the cyclecharacteristic of the lithium ion battery is prevented. However, eventhough the binder is used, it is difficult to completely suppress thedegradation of the active material. For this reason, the cyclecharacteristic cannot be effectively improved.

Solution to Problem

A charge/discharge control method of a lithium ion battery according tothe present invention is a charge/discharge control method of a lithiumion battery having a negative electrode active material and connected toa charge/discharge control device. In the charge/discharge controlmethod, battery information regarding a charge/discharge state of thelithium ion battery is acquired by the charge/discharge control device,a degradation state of the lithium ion battery is determined on thebasis of the battery information, by the charge/discharge controldevice, and a voltage range for charge/discharge of the lithium ionbattery is changed on the basis of a determination result of thedegradation state, by the charge/discharge control device.

A charge/discharge control device according to the present inventioncontrols charge/discharge of a lithium ion battery having a negativeelectrode active material. The charge/discharge control device includesa battery information acquisition unit which acquires batteryinformation regarding a charge/discharge state of the lithium ionbattery, a degradation state determination unit which determines adegradation state of the lithium ion battery, on the basis of thebattery information acquired by the battery information acquisitionunit, and a voltage range change unit which changes a voltage range forthe charge/discharge of the lithium ion battery, on the basis of adetermination result of the degradation state by the degradation statedetermination unit.

Advantageous Effects of Invention

According to the invention, a cycle characteristic of a lithium ionbattery can be effectively improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of acharge/discharge control device according to an embodiment of thepresent invention.

FIG. 2 is a diagram illustrating an example of a discharge curve showinga relation of a discharge capacity Q and a measurement result of abattery voltage V corresponding to the discharge capacity.

FIG. 3 is a diagram illustrating an example of a Q-dV/dQ curve.

FIG. 4 is a diagram illustrating an example of a Q-dV/dQ curvecalculated before degradation of a battery and a cycle test range.

FIG. 5 is a diagram illustrating a change example of a Q-dV/dQ curvecalculated in a state in which a battery is degraded and a cycle testrange.

FIG. 6 is a flowchart of a charge/discharge control process executed bya charge/discharge control device.

FIG. 7 is a diagram illustrating an example of a charge/discharge curvecorresponding to a battery of an initial state and a charge/dischargecurve corresponding to a battery of a degradation state.

FIG. 8 is a diagram illustrating a configuration example of an electrodebody configuring a battery.

FIG. 9 is a diagram illustrating an aspect where an electrode body isinterposed between sheets.

FIG. 10 is a diagram illustrating an aspect where sheets are thermallywelded.

FIG. 11 is a table illustrating a cycle test result.

FIG. 12 is a table illustrating the details of a cycle test resultaccording to a first example.

FIG. 13 is a table illustrating the details of a cycle test resultaccording to a second example.

FIG. 14 is a table illustrating the details of a cycle test resultaccording to a first comparative example.

DESCRIPTION OF EMBODIMENTS

(Configuration of Charge/Discharge Control Device)

FIG. 1 is a block diagram illustrating a configuration of acharge/discharge control device 100 according to an embodiment of thepresent invention. The charge/discharge control device 100 illustratedin FIG. 1 is a device to control charge/discharge of a lithium ionbattery 10 (hereinafter, simply referred to as the battery 10) to be asecondary battery and functionally has a battery information acquisitionunit 12, a degradation state determination unit 13, a voltage rangechange unit 14, and a control signal transmission unit 15. Thecharge/discharge control device 100 is connected to the battery 10 and acontroller 11.

The battery information acquisition unit 12 acquires information, suchas an inter-terminal voltage (closed circuit voltage) of the battery 10during the charge/discharge, a current flowing to the battery 10, and acharge/discharge time of the battery 10, as battery informationregarding a charge/discharge state of the battery 10. The batteryinformation acquisition unit 12 can acquire the battery informationusing a voltmeter, an ammeter, and a timer, for example. The batteryinformation acquisition unit 12 outputs each acquired batteryinformation to the degradation state determination unit 13.

The degradation state determination unit 13 determines a degradationstate, that is, a degradation degree of the battery 10, on the basis ofeach battery information acquired by the battery information acquisitionunit 12. A method of determining the degradation state of the battery 10by the degradation state determination unit 13 will be described indetail later. If the degradation state determination unit 13 determinesthe degradation state of the battery 10, the degradation statedetermination unit 13 outputs a determination result thereof to thevoltage range change unit 14.

The voltage range change unit 14 changes a voltage range for thecharge/discharge of the battery 10, on the basis of the determinationresult of the degradation state of the battery 10 by the degradationstate determination unit 13. That is, values to reset an upper limitvoltage (charge upper limit voltage) of the battery 10 at the time ofthe charge and a lower limit voltage (discharge lower limit voltage) ofthe battery 10 at the time of the discharge, preset in the controller11, according to the degradation state of the battery 10, arecalculated. A method of changing the voltage range by the voltage rangechange unit 14 will be described in detail later. If the voltage rangechange unit 14 calculates the resetting value of the charge upper limitvoltage and the resetting value of the discharge lower limit voltage,the voltage range change unit 14 outputs the resetting values asinformation showing a voltage range after the change to the controlsignal transmission unit 15.

If the voltage range for the charge/discharge of the battery 10 ischanged by the voltage range change unit 14, the control signaltransmission unit 15 transmits a control signal to command the voltagerange after the change to the controller 11. That is, control signalsshowing the resetting value of the charge upper limit voltage and theresetting value of the discharge lower limit voltage are generated andare outputted to the controller 11.

The charge/discharge control device 100 can execute charge/dischargecontrol of the battery 10 using the controller 11, by each configurationdescribed above.

The controller 11 controls an energization state of the battery 10during the charge/discharge, such that the charge/discharge of thebattery 10 is performed in ranges of the preset voltage and current. Ifthe control signals are transmitted from the control signal transmissionunit 15 of the charge/discharge control device 100 as described above,the controller 11 receives the control signals and changes the voltagerange for the charge/discharge of the battery 10, on the basis of thereceived control signals. In addition, energization to the battery 10 isperformed according to the voltage range after the change.

The charge/discharge control device 100 can realize the individualcomponents illustrated in FIG. 1, using a CPU, a ROM, a RAM, and an HDD,for example. That is, a process according to a predetermined programrecorded on the ROM or the HDD is executed by the CPU using the RAM, sothat individual functions of the battery information acquisition unit12, the degradation state determination unit 13, the voltage rangechange unit 14, and the control signal transmission unit 15 can berealized in the charge/discharge control device 100.

(Method of Determining Degradation State and Method of Changing VoltageRange)

Next, a method of determining a degradation state of the battery 10 bythe degradation state determination unit 13 and a method of changing thevoltage range for the charge/discharge of the battery 10 by the voltagerange change unit 14 will be described.

To determine the degradation state of the battery 10, first, thedegradation state determination unit 13 calculates a discharge capacityQ of the battery 10, that is, a total value of an amount of electricitydischarged from the battery 10 at the time of the discharge, for everypredetermined time, on the basis of a voltage, a current, and acharge/discharge time of the battery 10 shown by the battery informationfrom the battery information acquisition unit 12. Specifically, ameasurement value of a current obtained during the discharge of thebattery 10 is integrated for every predetermined time, so that thedischarge capacity Q can be calculated.

In this way, if the discharge capacity Q is calculated, the degradationstate determination unit 13 calculates dV/dQ showing a ratio of a changeamount dQ of the discharge capacity Q and a change amount dV of abattery voltage V, for every predetermined time. Specifically, thechange amount dQ of the discharge capacity Q and the change amount dV ofthe battery voltage V for every predetermined time are calculated from acalculation result of the discharge capacity Q obtained for everypredetermined time and individual measurement values of a voltage and acurrent at that time, so that dV/dQ can be calculated on the basis of acalculation result.

If the discharge capacity Q and dV/dQ of the battery 10 are calculatedfor every predetermined time as described above, the degradation statedetermination unit 13 calculates a Q-dV/dQ curve showing a relation ofthe discharge capacity Q and dV/dQ, on the basis of a calculationresult. Specifically, the Q-dV/dQ curve can be calculated by setting ahorizontal axis to show a value of the discharge capacity Q, setting avertical axis to show a value of dV/dQ, and graphically showing valuescalculated for every predetermined time.

FIG. 2 is a diagram illustrating an example of a discharge curve showinga relation of the discharge capacity Q calculated by the degradationstate determination unit 13 and a measurement result of the batteryvoltage V corresponding to the discharge capacity. The discharge curveshows an aspect of a change of the battery voltage V when the battery 10is discharged from a full charge state (discharge capacity 0) to aperfect discharge state (maximum discharge capacity Qmax) in which thedischarge is disabled. From FIG. 2, it is seen that, when the battery 10is discharged and the discharge capacity Q increases, the batteryvoltage V decreases.

FIG. 3 is a diagram illustrating an example of the Q-dV/dQ curvecalculated by the degradation state determination unit 13. The Q-dV/dQcurve illustrates an example of a relation of the discharge capacity Qand dV/dQ when the battery 10 is discharged from the full charge state(discharge capacity 0) to the perfect discharge state (maximum dischargecapacity Qmax).

In the Q-dV/dQ curve of FIG. 3, four characteristic singular points(peak points) shown by A, B, C, and D appear at individual positions ofdischarge capacities Q_(A), Q_(B), Q_(C), and Q_(D). The positions ofthe singular points and the number thereof are determined according tokinds of an electrode material and an active material of the battery 10.For example, when SiO (silicon oxide) is used as a negative electrodeactive material and a material obtained by mixing SiO with graphite isused for a negative electrode of the battery 10, the Q-dV/dQ curveillustrated in FIG. 2 can be obtained. In this case, the singular pointsB, C, and D show that the graphite to be the negative electrode materialcontributes to a discharge reaction in the negative electrode and thesingular point A shows that SiO to be the negative electrode activematerial contributes to the discharge reaction in the negativeelectrode. That is, in a range of the discharge capacity Q equal to orsmaller than Q_(B), lithium ions are discharged from the graphite and ina range of the discharge capacity Q larger than Q_(B), the lithium ionsare discharged from SiO.

If the Q-dV/dQ curve illustrated in FIG. 3 is calculated, thedegradation state determination unit 13 determines the degradation stateof the battery 10 on the basis of the Q-dV/dQ curve, as follows.

FIG. 4 is a diagram illustrating an example of a Q-dV/dQ curvecalculated before the degradation of the battery 10 and a cycle testrange. In FIG. 4, an example of the Q-dV/dQ curve calculated in a statebefore the degradation of the battery 10 when a range of a state ofcharge (SOC) of the battery 10 to be 25% to 75% is set as the cycle testrange and the charge/discharge of the battery 10 is continuouslyrepeated in the range is illustrated. In the Q-dV/dQ curve of FIG. 4,singular points A₁, B₁, C₁, and D₁ before the degradation appearing atindividual positions of discharge capacities Q_(A1), Q_(B1), Q_(C1), andQ_(D1) correspond to the singular points A, B, C, and D of FIG. 3. Inaddition, a maximum discharge capacity Qmax1 when the SOC is 0%corresponds to the maximum discharge capacity Qmax in the perfectdischarge state of FIG. 3. During the charge/discharge of the battery10, only a Q-dV/dQ curve of a portion corresponding to the cycle testrange in the drawing is calculated in the degradation statedetermination unit 13.

FIG. 5 is a diagram illustrating a change example of a Q-dV/dQ curvecalculated in a state in which the battery 10 is degraded and a cycletest range. If the charge/discharge is repeated and the battery 10 isdegraded, the Q-dV/dQ curve calculated changes according to thedegradation and a position of each singular point moves gradually in aleftward direction of the drawing. As a result, as illustrated in FIG.5, a singular point B₂ after the degradation may be included in thecycle test range before the change. In FIG. 5, singular points A₂, B₂,C₂, and D₂ after the degradation appearing at individual positions ofdischarge capacities Q_(A2), Q_(B2), Q_(C2), and Q_(D2) correspond tothe singular points A₁, B₁, C₁, and D₁ before the degradationillustrated in FIG. 4. In addition, a maximum discharge capacity Qmax2after the degradation corresponds to the maximum discharge capacityQmax1 before the degradation illustrated in FIG. 4 and shows the samevalue of dV/dQ.

In the above state, if the charge/discharge of the battery 10 isrepeated without changing the cycle test range, SiO to be the negativeelectrode active material contributes to the charge/discharge reactionin the negative electrode, so that SiO is degraded, and the degradationof the battery 10 is accelerated.

Therefore, when the Q-dV/dQ curve illustrated in FIG. 5 is calculated,the degradation state determination unit 13 determines that SiO to bethe negative electrode active material contributes to thecharge/discharge reaction in the negative electrode in the battery 10and determines that the battery 10 is in a degradation state. Adetermination result is output from the degradation state determinationunit 13 to the voltage range change unit 14. As a result, a warningshowing that the battery 10 is in the degradation state and it isnecessary to change a voltage range for the charge/discharge of thebattery 10 is output from the degradation state determination unit 13 tothe voltage range change unit 14.

In the degradation state determination unit 13, the degradation state ofthe battery 10 can be determined by the method described above.

If the voltage range change unit 14 receives the determination resultshowing that the battery 10 is in the degradation state, from thedegradation state determination unit 13, the voltage range change unit14 executes recalculation of the SOC, on the basis of the Q-dV/dQ curveof FIG. 5. Specifically, as illustrated in FIG. 5, a relation of thedischarge capacity Q and the SOC is recalculated such that the SOCbecomes 0% at the maximum discharge capacity Qmax2 after thedegradation. A range of 25% to 75% is specified in the SOC after thechange, on the basis of a result of the recalculation of the SOC, sothat the cycle test range can be changed.

If the recalculation of the SOC is executed as described above and thecycle test range is changed on the basis of the result thereof, thevoltage range change unit 14 calculates resetting values of a chargeupper limit voltage and a discharge lower limit voltage for the battery10, according to the cycle test range after the change. Specifically, arelation of the SOC and the battery voltage V in the battery 10 afterthe degradation is calculated on the basis of battery informationacquired by the battery information acquisition unit 12 when the Q-dV/dQcurve of FIG. 5 is calculated. The individual battery voltages V whenthe SOC is 75% and 25% are calculated on the basis of the relation, sothat the resetting values of the charge upper limit voltage and thedischarge lower limit voltage can be calculated.

In the voltage range change unit 14, the voltage range for thecharge/discharge of the battery 10 can be changed by the methoddescribed above. A relation of a discharge capacity and a voltage and aQ-dV/dQ curve of the positive electrode and the negative electrode maybe calculated by a method disclosed in Japanese Unexamined PatentPublication No. 2009-80093, for example, and the degradation state ofthe battery 10 may be determined using the Q-dV/dQ curve of the positiveelectrode and the negative electrode.

(Charge/Discharge Control Process)

Next, a charge/discharge control process executed by thecharge/discharge control device 100 when charge/discharge control of thebattery 10 is executed will be described. FIG. 6 is a flowchart of thecharge/discharge control process executed by the charge/dischargecontrol device 100.

In step S101, the charge/discharge control device 100 acquires batteryinformation regarding a charge/discharge state of the battery 10, by thebattery information acquisition unit 12. Here, as described above, theinter-terminal voltage, the current, and the charge/discharge time ofthe battery 10 are acquired as the battery information. The batteryinformation acquisition unit 12 transmits the acquired batteryinformation to the degradation state determination unit 13.

In step S102, the charge/discharge control device 100 calculates thedegradation state of the battery 10, by the degradation statedetermination unit 13, on the basis of the battery information acquiredby step S101. Here, the Q-dV/dQ curve is calculated from the batteryinformation by the method described above, so that the degradation stateof the battery 10 is calculated.

In step S103, the charge/discharge control device 100 determines whetherthe battery 10 is in the degradation state, by the degradation statedetermination unit 13, on the basis of the Q-dV/dQ curve acquired by thecalculation of the degradation state of step S102. Here, as described inFIG. 5, it is determined whether the specific singular point (singularpoint B2 of FIG. 5) showing that SiO to be the negative electrode activematerial contributes to the charge/discharge reaction in the negativeelectrode has been detected in the charge/discharge range of the battery10 in the Q-dV/dQ curve, so that it is determined whether the battery 10is in the degradation state. As a result, when it is determined that thebattery is in the degradation state, a determination result thereof istransmitted from the degradation state determination unit 13 to thevoltage range change unit 14, the warning for the change of the voltagerange is output, and the process proceeds to step S104. Meanwhile, whenit is determined that the battery is not in the degradation state, theprocess returns to step S101. In this case, after a predeterminedwaiting time, the processes of steps S101 to S103 are executed again.

The condition to determine that the battery 10 is in the degradationstate in step S103 may be set as other condition. For example, in theQ-dV/dQ curve illustrated in FIG. 5, when an intermediate point of thedischarge capacities Q_(A2) and Q_(B2) corresponding to the singularpoints A₂ and B₂ is taken and the intermediate point is included in thecharge/discharge range of the battery 10, it is determined that thebattery 10 is in the degradation state. Even in this case, it has beenconfirmed that the degradation of SiO to be the negative electrodeactive material does not progress as much and the degradation of thebattery 10 is suppressed. In addition, it can be determined whether thebattery 10 is in the degradation state, using various conditions, on thebasis of the Q-dV/dQ curve calculated by step S102.

In step S104, the charge/discharge control device 100 executes therecalculation of the SOC, by the voltage range change unit 14, on thebasis of the determination result of the degradation state of step S103.Here, as described in FIG. 5, the relation of the discharge capacity Qand the SOC is recalculated according to the degradation state of thebattery 10, using the Q-dV/dQ curve calculated by step S102.

In step S105, the charge/discharge control device 100 changes the chargeupper limit voltage and the discharge lower limit voltage for thebattery 10, by the voltage range change unit 14, on the basis of theresult of the recalculation of the SOC of step S104. Here, the range ofthe charge/discharge voltage of the battery 10 is reset according to therelation of the discharge capacity Q and the SOC recalculated by stepS104 and the resetting values of the charge upper limit voltage and thedischarge lower limit voltage are calculated according to the range. Atthis time, only any one of the charge upper limit voltage and thedischarge lower limit voltage may be changed. The voltage range changeunit 14 transmits the calculated resetting values to the control signaltransmission unit 15. As a result, control signals according to thecharge upper limit voltage and the discharge lower limit voltage afterthe change are transmitted from the control signal transmission unit 15to the controller 11 and the range of the charge/discharge voltage ofthe battery 10 is changed.

If step S105 is executed, the charge/discharge control device 100 endsthe charge/discharge control process of FIG. 6. In addition, thecharge/discharge control process of FIG. 6 is executed again from stepS101, after the predetermined waiting time.

When the voltage range change unit 14 receives the warning output fromthe degradation state determination unit 13 by step S103, the voltagerange change unit 14 may cause a user to select whether or not to changethe range of the charge/discharge voltage of the battery 10. In thiscase, when the user selects change of the range of the charge/dischargevoltage, the processes of steps S104 and S105 are executed in thevoltage range change unit 14. Meanwhile, when the user does not thechange of the range of the charge/discharge voltage, thecharge/discharge control process of FIG. 6 ends without executing theprocesses of steps S104 and S105 in the voltage range change unit 14.

Here, the change of the charge upper limit voltage will be described.The lithium ion battery is normally used in the range of the SOC ofabout 25% to 75%. However, the SOC is a value defined by the user andcannot be actually measured. For this reason, in normal control of thebattery 10 by the controller 11, the charge/discharge of the battery 10is controlled on the basis of the preset charge/discharge curve, suchthat the upper limit value and the lower limit value of the batteryvoltage V during the charge/discharge become values corresponding to theused SOC range of the battery 10, for example, the SOC range of 25% to75%. Specifically, when the battery 10 is used in the SOC range of 25%to 75%, the discharge lower limit voltage is a voltage corresponding toSOC=25% and the charge upper limit voltage is a voltage corresponding toSOC=75%.

As described above, if the battery 10 is degraded, voltagescorresponding to SOC values change (increase) even though the SOC valuesare the same. For this reason, if the charge/discharge of the battery 10is performed with the same charge upper limit voltage and dischargelower limit voltage as the charge upper limit voltage and the dischargelower limit voltage before the degradation though the battery 10 isdegraded, the actual degradation state of the battery 10 is notconsidered at all. For this reason, the life of the battery 10 may beshortened. In addition, the charge/discharge capability of the battery10 cannot be sufficiently utilized.

Therefore, in this embodiment, the charge upper limit voltage and thedischarge lower limit voltage are changed according to the degradationstate of the battery 10, by the charge/discharge control device 100.This point will be described specifically below with reference to FIG.7.

FIG. 7 is a diagram illustrating an example of a charge/discharge curvecorresponding to the battery 10 of the initial state and acharge/discharge curve corresponding to the battery 10 of thedegradation state. In FIG. 7, portions corresponding to SOC=50% to 100%are enlarged and individual charge/discharge curves are illustrated.

In the initial state in which the battery 10 is not degraded, asillustrated in FIG. 7, a value of the battery voltage V corresponding toSOC=70% is V1. Therefore, the charge upper limit voltage is set as V1,so that the charge can be performed until the SOC becomes 70%.Meanwhile, when the battery 10 is degraded and enters the degradationstate, as illustrated in FIG. 7, the value of the battery voltage Vcorresponding to SOC=70% increases from V1 to V2. For this reason, ifthe charge is performed with the charge upper limit voltage as V1,according to the charge/discharge curve of the initial state, withoutconsidering the degradation state of the battery 10, the SOC increasesto only 55%, not 70%. That is, the charge ends though there stillremains a margin in the capacity of the battery 10.

Therefore, the charge upper limit voltage is changed according to thedegradation state of the battery 10, by the charge/discharge controldevice 100, to avoid such a situation. That is, when the battery 10 isdegraded, the charge upper limit voltage changes from V1 to V2 accordingto the degradation. At this time, it is not necessary to change thecharge upper limit voltage to V2 and the charge upper limit voltage maybe changed to approximate to V2. In this way, even though the battery 10is degraded, the charge does not end when the SOC is 55% and the chargecan be performed until the SOC becomes about 70%, similar to the initialstate. Therefore, even though the battery 10 is degraded, a decrease ofthe capacity of the battery 10 to be usable can be avoided.

In the above description, the change of the charge upper limit voltagehas been described. This is the same in the change of the dischargelower limit voltage. That is, the discharge lower limit voltage ischanged according to the degradation state of the battery 10, so thatthe decrease of the capacity of the battery 10 to be usable can beavoided even though the battery 10 is degraded.

(Configuration of Battery)

Next, a configuration of the battery 10 will be described with referenceto FIGS. 8 to 10. Hereinafter, a configuration example of the battery 10using a rocking chair type lithium ion secondary battery will bedescribed. FIG. 8 is a diagram illustrating a configuration example ofan electrode body 4 configuring the battery 10. FIG. 9 is a diagramillustrating an aspect where the electrode body 4 is interposed betweensheets 7. FIG. 10 is a diagram illustrating an aspect where the sheets 7are thermally welded.

As illustrated in FIG. 8, the electrode body 4 has a positive electrode1, a negative electrode 2, and a bag-shaped separator 3. The positiveelectrode 1 is connected to a positive electrode terminal 5 and thenegative electrode 2 is connected to a negative electrode terminal 6.

For the positive electrode 1, a resultant obtained by mixing N-methylpyrrolidone (NMP) as a solvent such that a layered LiMO2 (M shows Ni0.5Co0.2 Mn0.3) functioning as a positive electrode active material,acetylene black functioning as a conductive material, and polyvinylidenefluoride (PVdF) functioning as a binder become a ratio of weight of93:4:3 is used as positive electrode mixture slurry. After the positiveelectrode mixture slurry is coated on an aluminum foil having athickness of 15 μm and is dried in the atmosphere, a resultant is moldedwith a size of 45 mm×70 mm by a roll press and is cut in a shapeincluding a current collection foil exposure portion. In this way, thepositive electrode 1 is manufactured.

For the negative electrode 2, a resultant obtained by mixing water as asolvent such that graphite, SiO, carboxymethyl cellulose (CMC)functioning as a binder, and styrene butadiene rubber (SBR) become aratio of weight of 93:5:1:1 is used as negative electrode mixtureslurry. After the negative electrode mixture slurry is coated on acopper foil having a thickness of 10 μm and is dried in the atmosphere,a resultant is molded with a size of 45 mm×70 mm by a roll press and iscut in a shape including a current collection foil exposure portion. Inthis way, the negative electrode 2 is manufactured.

As the separator 3, film materials in which polypropylene, polyethylene,and polypropylene are stacked in three layers and a total thickness is0.03 mm are used. Two film materials are used, the positive electrode 1is interposed between the film materials, surrounding three sides arethermally welded to make a bag shape, and the separator 3 ismanufactured.

In a state in which the positive electrode 1 and the negative electrode2 are inserted into the bag-shaped separator 3 and the currentcollection foil exposure portions are exposed to the outside of theseparator 3, the positive electrode terminal 5 and the negativeelectrode terminal 6 are connected by ultrasonic welding, so that theelectrode body 4 illustrated in FIG. 8 is manufactured.

Next, as illustrated in FIG. 9, the electrode body 4 is arranged betweenthe two sheets 7 to be thermally welded and is interposed between thesheets 7. In this state, as illustrated in FIG. 10, the two sheets 7 arethermally welded in a thermal welding portion 8, except for a pouringhole (not illustrated in the drawings) to pour an electrolyte. After theelectrolyte is poured from the pouring hole, the pouring hole is sealedby thermal welding.

As the electrolyte of the battery 10, an organic solvent obtained bymixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), anddimethyl carbonate (DMC) to become a volume ratio of 1:2:2 is used. Aresultant by dissolving lithium hexafluorophosphate (LiPF6) in theorganic solvent to become 1.0 mol/L is used as the electrolyte of thebattery 10.

After the electrolyte is poured and the pouring hole is sealed, anelectrolyte impregnation time of 8 hours is set. Then, the battery 10 isfinished by performing 3 cycle discharge/discharge by a current value of0.2 CA, in a voltage range of 4.2 V to 2.5 V.

(Cycle Test)

Next, a result obtained by executing a cycle test of the battery 10 bythe configuration illustrated in FIG. 1 using the battery 10manufactured as described above will be described. When the cycle testis executed, values of the battery capacity and the internal resistancein the initial state are measured previously as characteristic data ofthe battery 10 before the degradation and a charge/discharge curve whenthe battery 10 is charged/discharged with 0.02 CA is acquired.

The used SOC range of the battery 10 is set as 25% to 75% and thecharge/discharge is repeated until a capacity retention rate of thebattery 10 decreases to about 80%, so that the cycle test is executed.FIG. 11 is a table illustrating a cycle test result. In FIG. 11, a cycletest result when the voltage range of the charge/discharge is changedtwo times according to the degradation state of the battery 10 duringthe cycle test is shown as a first example and a cycle test result whenthe voltage range is changed once is shown as a second example. Inaddition, a cycle test result when the voltage range of thecharge/discharge is not changed is shown as a first comparative example.

As illustrated in FIG. 11, in the first comparative example in which thevoltage range of the charge/discharge is not changed, 4000charge/discharge cycles are executed until the cycle test ends and inthe first and second examples, 5000 and 4500 charge/discharge cycles areexecuted, respectively. In addition, if an integration capacity in thefirst comparative example, that is, an integration value of thedischarge capacity during the cycle test is set as 1, in both the firstand second examples, an integration capacity of 1.1 times is obtained.In other words, from the cycle test results, it can be seen that thevoltage range for the charge/discharge of the battery 10 is changedaccording to the degradation state thereof, using the charge/dischargecontrol method according to the present invention, so that a cyclecharacteristic of the battery 10 can be effectively improved.

FIG. 12 is a table illustrating the details of the cycle test resultaccording to the first example. As illustrated in FIG. 12, in the firstexample, when the charge/discharge cycle numbers are 2000 and 4000, itis determined that the battery 10 is in the degradation state, so thatthe SOC is recalculated and the voltage range of the charge/discharge ischanged. As a result, SiO to be the negative electrode active materialcontributes to the charge/discharge reaction in the negative electrode,so that the degradation of the battery 10 is suppressed, and 5000charge/discharge cycles can be executed until the capacity retentionrate of the battery 10 decreases to about 80%.

FIG. 13 is a table illustrating the details of the cycle test resultaccording to the second example. As illustrated in FIG. 13, in thesecond example, when the charge/discharge cycle number is 3000, it isdetermined that the battery 10 is in the degradation state, so that theSOC is recalculated and the voltage range of the charge/discharge ischanged. As a result, SiO to be the negative electrode active materialcontributes to the charge/discharge reaction in the negative electrode,so that the degradation of the battery 10 is suppressed, and 4500charge/discharge cycles can be executed until the capacity retentionrate of the battery 10 decreases to about 80%.

FIG. 14 is a table illustrating the details of the cycle test resultaccording to the first comparative example. As illustrated in FIG. 14,in the first comparative example, the charge/discharge of the battery 10is continuously performed without changing the voltage range of thecharge/discharge. As a result, the discharge/discharge cycle numberuntil the capacity retention rate of the battery 10 decreases to about80% is 4000 smaller than the discharge/discharge cycle numbers of thefirst and second examples.

According to one embodiment of the present invention described above,the following functions and effects can be achieved.

(1) The charge/discharge control device 100 controls thecharge/discharge of the battery 10 that has SiO to be the negativeelectrode active material. The charge/discharge control device 100includes the battery information acquisition unit 12 that acquires thebattery information regarding the charge/discharge state of the battery10, the degradation state determination unit 13 that determines thedegradation state of the battery 10, on the basis of the batteryinformation acquired by the battery information acquisition unit 12, andthe voltage range change unit 14 that changes the voltage range for thecharge/discharge of the battery 10, on the basis of the determinationresult of the degradation state by the degradation state determinationunit 13. Therefore, the cycle characteristic of the battery 10 can beeffectively improved.

(2) The degradation state determination unit 13 calculates the Q-dV/dQcurve showing the relation of the discharge capacity Q of the battery 10and dV/dQ showing the ratio of the change amount dV of the batteryvoltage V to the change amount dQ of the discharge capacity Q, on thebasis of the battery information acquired by the battery informationacquisition unit 12 (step S102), and determines the degradation state ofthe battery 10, on the basis of the Q-dV/dQ curve (step S103).Therefore, the degradation state of the battery 10 can be determinedaccurately.

(3) The voltage range change unit 14 executes the SOC recalculation torecalculate the relation of the discharge capacity Q of the battery 10and the state of charge (SOC) of the battery 10, on the basis of thedetermination result of the degradation state by the degradation statedetermination unit 13 (step S104), and changes the voltage range for thecharge/discharge of the battery 10, on the basis of the result of theSOC recalculation (step S105). Therefore, the voltage range for thecharge/discharge of the battery 10 can be appropriately changedaccording to the degradation state of the battery 10.

In the embodiment described above, the Q-dV/dQ curve is calculated onthe basis of the acquired battery information, it is determined whetherthe specific singular point is included in the charge/discharge range ofthe battery 10 in the Q-dV/dQ curve, and the degradation state of thebattery 10 is determined. However, the degradation state of the battery10 may be determined by other method. For example, a position relationof the individual singular points in the Q-dV/dQ curve is calculated andthe degradation state of the battery 10 can be determined from theposition relation. The degradation state of the battery 10 can bedetermined by various methods, according to the kind of the activematerial or the electrode material used in the battery 10.

In addition, the configuration of the lithium ion battery controlled inthe charge/discharge control method and the charge/discharge controldevice according to the present invention is not limited to theconfiguration described in the embodiment. A specific configuration of abattery that includes a positive electrode capable ofstoring/discharging lithium ions, a negative electrode capable ofstoring/discharging the lithium ions, and lithium salt is not limited inparticular. For example, a battery using a nonaqueous electrolyte and abattery including a lithium ion polymer may be used. In addition, abattery including a solid electrolyte and a battery including an ionliquid may be used. The separator is not an essential configuration andmay be used if necessary.

The embodiments and the modifications described above are only exemplaryand the present invention is not limited to the above content, as longas a characteristic of the invention is not deteriorated.

REFERENCE SIGNS LIST

-   1 positive electrode-   2 negative electrode-   3 separator-   4 electrode body-   5 positive electrode terminal-   6 negative electrode terminal-   7 sheet-   8 thermal welding portion-   10 lithium ion battery-   11 controller-   12 battery information acquisition unit-   13 degradation state determination unit-   14 voltage range change unit-   15 control signal transmission unit-   100 charge/discharge control device

1. A charge/discharge control method for a lithium ion battery having anegative electrode active material, the lithium ion battery beingconnected to a charge/discharge control device, the method comprising:acquiring battery information regarding a charge/discharge state of thelithium ion battery by the charge/discharge control device; calculatinga Q-dV/dQ curve showing a relationship of discharge capacity Q of thelithium ion battery and dV/dQ, which shows a ratio of a change amount dVof a battery voltage V to a change amount dQ of the discharge capacity Qon the basis of the battery information, by the charge/discharge controldevice; determining a degradation state of the lithium ion battery onthe basis of the information-dV/dQ curve, by the charge/dischargecontrol device; and changing a voltage range for charge/discharge of thelithium ion battery on the basis of a determination result of thedegradation state, by the charge/discharge control device.
 2. (canceled)3. The charge/discharge control method of the lithium ion batteryaccording to claim 1, wherein SOC recalculation to recalculate arelationship of the discharge capacity Q of the lithium ion battery anda state of charge SOC of the lithium ion battery is executed on thebasis of the determination result of the degradation state, by thecharge/discharge control device, and the voltage range is changed on thebasis of a result of the SOC recalculation, by the charge/dischargecontrol device.
 4. A charge/discharge control device for controllingcharge/discharge of a lithium ion battery having a negative electrodeactive material, comprising: a battery information acquisition unitwhich acquires battery information regarding a charge/discharge state ofthe lithium ion battery; a degradation state determination unit whichdetermines a degradation state of the lithium ion battery, on the basisof the battery information acquired by the battery informationacquisition unit; and a voltage range change unit which changes avoltage range for charge/discharge of the lithium ion battery, on thebasis of a determination result of the degradation state by thedegradation state determination unit wherein the degradation statedetermination unit calculates a Q-dV/dQ curve showing a relationship ofdischarge capacity Q of the lithium ion battery and dV/dQ, which shows aratio of a change amount dV of a battery voltage V to a change amount dQof the discharge capacity Q, on the basis of the battery information,and determines the degradation state of the lithium ion battery, on thebasis of the Q-dV/dQ curve.
 5. (canceled)
 6. The charge/dischargecontrol device according to claim 4, wherein the voltage range changeunit executes SOC recalculation to recalculate a relation of thedischarge capacity Q of the lithium ion battery and a state of chargeSOC of the lithium ion battery, on the basis of the determination resultof the degradation state, and changes the voltage range, on the basis ofa result of the SOC recalculation.